Using a pestle and mortar, pound coriander roots, salt, garlic, and peppercorns into a fine paste. Cut chicken in half along the breastbone. Flatten out, wash and dry. Work in the paste, fish sauce and sugar and leave to marinate for a few hours.
For the sweet chili sauce, pound coriander root, salt, garlic and chiles into a paste. Combine vinegar, sugar, water, and salt in a pot and bring to the boil. Add paste and stir to dissolve. Simmer sauce until reduced by half, skimming as necessary, then set aside to cool.
Chargrill chicken for about 15-20 minutes, turning regularly. Serve with a bowl of the sauce.
Recipe courtesy of David Thompson, Thai Food, Ten Speed Press, August 1, 2002